Multicarrier transmission system with low power sleep mode and rapid-on capability

ABSTRACT

A muticarrier transceiver is provided with a sleep mode in which it idles with reduced power consumption when it is not needed to transmit or receive data. The full transmission and reception capabilities of the transceiver are quickly restored when needed, without requiring the full (and time-consuming) initialization commonly needed to restore such transceivers to operation after inactivity.

BACKGROUND OF THE INVENTION

The invention relates to multicarrier transmission systems, andcomprises method and apparatus for establishing a power management sleepstate in a multicarrier system.

SUMMARY OF THE INVENTION

This application is based on United States Provisional Application Ser.No. ______, filed Jan. 26, 1998 by John Greszczuck, Rick Gross and HalilPadir and claims the benefit thereof.

Multicarrier transmission systems provide high speed data links betweencommunication points. Such systems have recently been introduced forcommunications over the local subscriber loop that connects a telephoneservice subscriber to a central telephone office; in this importantapplication they are commonly referred to as “xDSL” systems, where the“x” specifies a particular variant of DSL (digital subscriber loop)communications, e.g., ADSL (asynchronous digital subscriber loop), HDSL(High-Speed Digital Subscriber Loop), etc. These will be referred togenerically herein simply as “DSL” systems.

In such systems, a pair of transceivers communicate with other bydividing the overall bandwidth of the channel interconnecting thesubscriber and the central office into a large number of separatesubchannels, each of limited bandwidth, operating in parallel with eachother. For example, one common system divides the subscriber linechannel into two hundred and fifty six subchannels, each of 4.3kilohertz bandwidth. A first group of these (e.g., one hundred ninetysix) is allocated to communications from the central office to thesubscriber (this is known as the “downstream” direction); a second group(e.g., thirty-two) is allocated to communications from the subscriber tothe central office (this is known as the “upstream” direction). Theremaining subchannels are allocated to administrative, overhead andcontrol (AOC) functions.

Data to be communicated over the link is divided into groups of bits,one group for each subchannel. The group of bits allocated to a givensubchannel is modulated onto a carrier whose frequency is specific tothat channel. Typically, quadrature amplitude modulation (QAM) is usedfor this purpose, and the group of bits is mapped into a vector definedby one of the points of a “constellation” which specifies the allowabledata points for transmission over that subchannel at a particular time.Each vector or data point thus comprises a unique symbol representing aspecific bit configuration for transmission as a group over itsassociated subchannel. During the time period allocated for transmissionof a symbol (commonly referred to as a “symbol period” or “frame”), eachsubchannel transmits its symbol in parallel with all other subchannelsso that large amounts of data can be transmitted during each frame.

The number of bits carried by a symbol is dependent on thecharacteristics of the subchannel over which it is to be transmitted.This may vary from one subchannel to another. The principal determinantis the signal-to-noise ratio of the subchannel. Accordingly, thisparameter is measured from time to time in order to ascertain its valuefor each subchannel, and thus determine the number of bits to betransmitted on the particular subchannel at a given time.

The telephone channel is subject to a number of impairments which mustbe compensated for in order to ensure reliable transmission. Phase(delay) distortion of the transmitted signal is typically the mostlimiting of these impairments. This distortion is frequency-dependent,and thus components of a signal at different frequencies are shifted byvarying amounts, thereby distorting the signal and increasing thelikelihood of erroneous detection unless provision is made to combat it.To this end, frequency domain equalizers (FDQ) and time domainequalizers (TDQ) are commonly incorporated into the transmission channelin order to equalize the phase (time) delay across the channel frequencyband. Other impairments also exist. For example, frequency-dependentsignal attenuation adversely affects signal transmission on thetelephone line. This is compensated by the use of gain equalizers on theline, while echo on the line is handled by the use of echo cancellers.

The problem of signal impairment is especially serious in those xDSLconfigurations which carry the DSL communications on a common line withordinary voice communications but which omit the use of a “splitter” ateither the subscriber premises or the central office or both. A“splitter” is basically a filter which separates the low-frequency voicecommunications (e.g., from zero to four kilohertz) from thehigher-frequency data communications (which may extend up into themegahertz band) and provides a strong degree of isolation between thetwo. In the absence of a splitter, unique provisions must be made toaccommodate voice and data communications on the same line. For a moredetailed description of the problem and its solution, see the co-pendingapplication of Richard Gross et al. entitled “Splitterless MulticarrierModem”, Serial No. PCT/US98 21442, filed Oct. 9, 1998, and assigned tothe assignee of the present invention, the disclosure of which isincorporated herein by reference.

Because of their extensive use in Internet communications as well as inother applications, DSL transceivers are commonly maintained in the “on”state, ready to transmit or receive once they have been installed andinitialized. Thus, such modems consume a significant amount of power,even when they are not actively transmitting or receiving data. It isgenerally desirable to limit this power consumption, both forenvironmental reasons as well as to prolong the life of the equipment.Further, such modems may be implemented or incorporated in part or inwhole in computer equipment such as in personal computers for home andbusiness use, and such computers increasingly incorporate powerconservation procedures. See, for example, U.S. Pat. No. 5,428,790,“Computer Power Management System”, issued Jun. 27, 1995 on theapplication of L. D. Harper. Thus, it is desirable to provide an ADSLmodem which can accommodate power conservation procedures in equipmentwith which it is associated, as well as independently of such equipmentas may be appropriate.

Because of the complexity of DSL transceivers, and the conditions underwhich they must operate, it is necessary to initialize them prior to thetransmission and reception of data. This initialization includes, interalia, channel corrections such as “training” the frequency-domain andtime-domain equalizers and the echo cancellers; setting the channelgains; negotiating the transmission and reception data rates; adjustingthe fine gains on the subcahnnels over which communication is to takeplace; setting the coding parameters; and the like. Additionally, itincludes measuring the signal-to-noise ratio of each of the subchannels,calculating the bit-allocation tables characteristic of each under givenconditions of transmission, and exchanging these tables with othermodems with a given modem communicates. For more detailed discussion ofthese procedures, refer to the application of Richard Gross et al.,cited above and incorporated herein by reference. These procedures canrequire from seconds to tens of seconds. In a new installation, the timerequired is inconsequential. However, in an already-operatinginstallation, the time required to initialize or re-initialize thesystem after a suspension of operation in connection with powerconservation is generally unacceptable, since it is typically desired tohave the modem respond to request for service nearly instantaneously.

Accordingly, it is an object of the invention to provide a multicarriertransmission system having a low power sleep mode and a rapid-oncapability.

Further, it is an object of the invention to provide a multicarriertransmission system for use in digital subscriber line communicationsthat can rapidly switch from a sleep mode to a full-on condition.

Still another object of the invention is to provide a DSL system thatcan readily be integrated into a computer having a low power sleep modeand which is capable of rapid return to full operation.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention description below refers to the accompanying drawings, ofwhich:

FIG. 1 is a block and line diagram of a multicarrier transmission systemin accordance with a preferred embodiment of the present invention;

FIG. 1A is a portion of an exemplary chart showing a possible bitdistribution among subchannels;

FIG. 1B illustrates a timing signal used in accordance with theinvention;

FIG. 2 is a flow diagram of the operation of the present invention; and

FIG. 3 is a block and line diagram of still another aspect of thepresent invention.

DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT

For purposes of explanation, the present invention will be described inthe context of an ADSL system having a first transceiver located at thesite of a customer's premises (referred to hereinafter as the “CPEtransceiver”) and a second transceiver located at a local centraltelephone office (hereinafter referred to as the “CO transceiver”). Thetwo are interconnected for communication by means of a common telephoneline over which voice and data are to be transmitted, and the COtransceiver is commonly connected into a broader network such as theInternet to and from which data is to be communicated. The system willbe described as using Fourier transform technology for modulation anddemodulation of the data to be transmitted. It will be understood,however, that the invention is not limited to this environment, and isapplicable to point-to-point communications is other environments, andwith other forms of modulation/demodulation. Further, since the CPEtransceiver and CO transceiver are very similar, the invention will beexplained in connection with a detailed illustration of the CPEtransceiver only.

In FIG. 1, a DSL transceiver 10 in accordance with the present inventionhas a transmitter section 12 for transmitting data over a digitalsubscriber line 14 and a receiver section 16 for receiving data from theline. The transmitter section 12 is formed from an input buffer andconverter (IBC) 18 that receives a serial string of data (e.g., binarydigits) b_(i) to be transmitted and converts the data into a pluralityof pairs of complex-valued symbols X_(i) and their conjugatesX_(N-i)=X*_(i), i=0, 1, . . . N. Typically, the buffer 18 holds at leasta frame of data (a frame comprising the amount of data to be transmittedduring one symbol period). The pairs of symbols X_(i) and X*_(i) areapplied to an Inverse Fast Fourier Transform (IFFT) 20 to provide realtime output signals x_(j), j=0, 1, . . . N/2−1. The latter in turn areconverted to serial form in a parallel-to-serial converter (PSC) 22 andthen applied to a digital-to-analog converter (DAC) 24 for applicationto a line driver 26. The converter 24 may apply a cyclic prefix to thesignals x_(j) to combat intersymbol interference caused by thetransmission medium. The driver 26 may incorporate a gain controlsection (GC) 26 a for controlling the signal amplitude (and thus power)as it is applied to a communication channel such as the digitalsubscriber line 14.

IFFT 20 may be viewed as a data modulator. The symbols X_(i), and theirconjugates X_(N-i), correspond to data points defining signal vectors ina quadrature amplitude modulation (QAM) constellation set. The converter18 forms the respective symbols from the input data with the aid of abit allocation table (BAT) 28 which specifies, for each subchannel, thenumber of bits to be carried by the symbol transmitted over thatsubchannel, and thus defines the data point to be associated with thesymbol. This table is typically calculated at the transceiver and istransmitted to other transceivers with which the instant transceivercommunicates, to thereby enable them to decode the symbols received bythem from the instant transceiver.

The number of bits which each symbol carries is determined by thecharacteristics of the subchannel over which the symbol is to betransmitted, and particularly by the signal-to-noise ratio of thesubchannel. Procedures for this calculation are known. FIG. 1A shows anexample of such a table as formed and stored at transceiver 10. Thus,the symbol to be transmitted over subchannel 50 may be determined tohave an allocation of six bits; that of subchannel 51, six bits; that ofsubchannel 52, seven bits, etc.

A Clock 30 controls the timing of the operation of the transmitter 12.It supplies input to a Controller 32 which controls the individual unitsof the transmitter. In the case of the CO transceiver, the clock 30typically is a master clock to which a remote transceiver, such as at asubscriber premises, will be synchronized. In the case of a transceiverat the subscriber premises, such as is shown here for purposes ofillustration, the clock is derived from the master clock at the centraloffice as described more fully below in connection with the receiverportion of the transceiver.

A Frame Counter (FC) 34 connected to the controller 32 maintains a countof the number of frames of data transmitted from or received by thetransceiver 10. The clock 30 maintains the count in counter 34synchronous with that of a corresponding counter (not shown) in the COtransceiver. In DSL systems, typically, data is communicated in the formof a sequence of data frames (e.g., sixty-eight frames for ADSL asspecified in ITU Document G.992.2), followed by a synchronization frame,each frame having a duration of one symbol period of approximately twohundred and fifty microseconds. Together, the sixty-nine frames comprisea “superframe”. Thus, the counter 34 typically maintains a count modulosixty-nine. Finally, a State Memory (SM) 36 connected to the controller32 records the state of the transceiver for reasons discussed more fullybelow.

Turning now to the receiver section 16, it is formed from a lineconditioner 50; an analog-to-digital converter (ADC) 52; aserial-to-parallel converter 54; a Fast Fourier Transform (FFT) section56; a decoder 58; and a parallel-to-serial converter 60. The conditioner50 compensates for transmission distortions introduced by the line 14,and commonly includes a frequency-domain equalizer (FDQ) 50 a; atime-domain equalizer (TDQ) 50 b; and an echo canceller (EC) 50 c, amongother elements. The ADC 52 converts the received signal to digital formand applies it to the serial-to-parallel converter 54. The converter 54removes any cyclic prefix that may have been appended to the signalbefore it was transmitted, and applies the resultant signal to the FFT56 which effectively “demodulates” the received signal. The output ofthe FFT is applied to decoder 58 which, in conjunction with abit-allocation-table 62, recovers the symbols X_(i) and X_(i)* and thebits associated with them. The output of detector 58 is applied to theparallel-to-serial converter 60 which restores the data stream, b_(i),that was originally applied to the transmitter. The controller 32 alsocontrols the operation of the receiver portion 16 of the transceiver 10.

During normal (non-sleep mode) operation, a phase-lock loop (PLL) 62receives from the FFT 56 a timing reference signal 62 a (see FIG. 1A)via a line 62 b. The timing reference signal 62 a is transmitted fromthe transmitter with which the receiver 16 communicates (e.g., the COtransmitter). This signal is advantageously a pure tone of fixedfrequency and phase which is synchronized with the Master Clock in thetransmitter; its frequency defines the frame rate of the transceivers.Other forms of timing signal may, of course, be used, but use of a puretone has the advantage of simplicity and reliability even when portionsof the transceiver are powered down in accordance with the invention.The PLL 62 locks itself to this signal and drives clock 30 insynchronism with the Master Clock in the driving transmitter. This alsosynchronizes frame counter 34 of the CPE transceiver to thecorresponding frame counter of the CO transceiver. Control of thereceiver section is provided by the controller 32.

In the sleep mode, the FFT 56 is preferably dormant. Accordingly, thetiming reference signal for PLL 62 is provided from the output of theanalog to digital converter 52 via a detector 64 which extracts thetiming signal from the signal appearing on line 14 during sleep mode. bycalculating the DFT of the synchronizing pilot tone. Controller 32controls the switching of the input to PLL 62 between these two sourcesso that the PLL remains locked to the CO transceiver timing reference.

As noted earlier, the transceiver of the present invention will commonlybe incorporated in a computer such as a personal computer; indeed, itmay be implemented as an integral part of such a computer, which mayhave a power conservation capability for activation when the computer isnot in active operation. It is thus desirable that the transceiver beable to suspend operations and enter a “sleep” mode in which it consumesreduced power when it is not needed for data transmission or reception,but nonetheless be able to resume transmission or reception almostinstantaneously, e.g., within a few frames.

Further, when the ADSL transceiver is implemented as an integral part ofa computer, it may often be the case that the processing power of thecomputer is, at a given moment, devoted to another task such asgraphics, word processing, and the like, and is thus unable to servicethe transceiver. In such circumstances, it is possible that a frame thatis assembled for transmission to the CO transceiver, and thence to somenetwork connected to it, is incomplete, and thus would generate errorsif transmitted. Accordingly, the transceiver of the present invention isresponsive to such conditions by entering an “idle” state in which itceases active transmission of data while the computer is elsewhereoccupied. This state is similar in many ways to the sleep mode statealthough, of course, its purpose is not power conservation, and thus inthe idle state the power to selected portions of the transceiver may,but need not, be reduced. The idle state maintains synchronous signalingbetween the CPE and CO transceivers but no data is transmitted. Onreceipt of the idle indicator from the CPE, the CO transceiver transmitsidle cells to the network to maintain connection with the network.

Referring now more particularly to FIG. 2, the power down operation ofthe CPE transceiver begins on receipt of a power down indication (step80) by the CPE transceiver controller 32. The power down indication maybe applied to the controller 32 from an external source such as apersonal computer in which the transceiver is included; it may begenerated within the transceiver itself as a result of monitoring theinput buffer 18 and determining that no data has been applied to it fora given time interval or that the buffer has not been filled despitepassage of a symbol time; it may be responsive to a power down commandfrom the CO transceiver; or it may be generated in response to otherconditions.

Considering for the moment the first two cases, the CPE transceiverresponds to the indication by transmitting to the CO transceiver an“Intend To Enter Sleep Mode” notification (step 82). This notificationindicates that the transceiver is about to undergo a change of state,and may take any of a variety of forms; preferably it is a message, butmay also comprise a tone, an inverted sync signal, a flag, or even thecessation of data transmission itself. The notification mayadvantageously be transmitted over one of the embedded operationschannels of the system. It provides the CO transceiver an opportunity toprepare itself to enter sleep mode, and also to signal the CPEtransceiver if entrance into sleep mode is not appropriate or desirableat the particular moment.

If entrance into sleep mode is permissible at this time, the COtransceiver responds to the power down or idle signal by transmitting an“Acknowledge Sleep Mode” notification (step 84) to the CPE transceiver.This and subsequent notifications described in connection with the sleepor idle mode may similarly take any of a variety of forms such asdescribed above for the “Intend To Enter Sleep Mode” notification, butagain preferably is in the form of a message transmitted over anembedded operations channel.

After it has received acknowledgment from the CO transceiver, the CPEtransceiver transmits an “Entering Sleep Mode” notification (step 86) tothe CO transceiver and ceases transmission, either immediately or aftera given number of frames. The CO transceiver detects this notification;transmits its own “Entering Sleep Mode” notification (step 88); andenters sleep mode (step 90). In pursuance of this, the CO transceiverstores its state in its own state memory corresponding to the statememory 38 of CPE transceiver 10. The state of the CO or CPE transceiverspreferably includes at least the frequency and time-domain equalizercoefficients (FDQ; TDQ) and the echo-canceller coefficients (ECC) of itsreceiver portion and the gain of its transmitter portion; thetransmission and reception data rates; the transmission and receptioncoding parameters; the transmission fine gains; and the Bit AllocationTables. The CO transceiver continues to advance the frame count andsuperframe count during the period of power-down in order to ensuresynchrony with the remote CPE transceiver when communications areresumed. In order to maintain synchronization during the power down oridle state, the CO transceiver continues to transmit to the CPEtransceiver the synchronizing pilot tone 62 a It may, at this time,perform its own power reduction. In particular, it may reduce or cut offpower to the digital modulator/demodulator portions of its transmitterand receiver sections (corresponding to the IFFT 20 and FFT 56 of theCPE transceiver, FIG. 1); this provides a significant power reduction.Further, it may reduce power to parts of the analog circuitry. Powerwill be maintained, of course, to at least that portion of the analogdriver circuitry which transmits the pilot tone and other controlsignals to the CPE transceiver, and to line circuits required to monitorthe line 14 for signals from the CPE transceiver.

In a central office, the rest of the equipement is still operating, andwhile the link between the CO transceiver and the CPE transceiver is ina sleep state, user data provided by the CO transceiver will be benignidle data such as ATM Idle Cells or HDLC Flag octets, and not generatefalse data errors to other equipment.

In response to the “Entering Sleep Mode” notification from the COtransceiver, the CPE transceiver enters the sleep mode (step 92). Inparticular, it stores its state (step 94) in state memory 38; as notedabove in connection with the CO transceiver, this includes preferably atleast the frequency and time-domain equalizer coefficients (FDQ; TDQ)and the echo-canceller coefficients (ECC) of its receiver and the gainof its transmitter; the transmission and reception data rates; thetransmission and reception coding parameters; the transmission finegains; and the Bit Allocation Tables. The phase and frequency offset ofthe phase-locked loop 62 is maintained by continued opertion of theloop. The CPE transceiver 10 then reduces power to the digitalmodulator/demodulator circuitry comprising IFFT 20 and FFT 56, as wellas to and transmitter data line drivers 26. However, it continues toadvance the frame counter 34 in accordance with the receivedsynchronizing signal 62 a. However, the CPE controller 32 now causesthis signal to be applied to the PLL 62 from the output of the ADC 52(FIG. 1) via the detector 64, which implements the DFT of a single tone,instead of directly from the output of the FFT 56 as was previously thecase. This enables the FFT 56 to be powered down. The CPE and COtransceivers then operate in sleep mode (steps 95 and 97, respectivley)until they awaken.

During the sleep mode state, the CO transceiver continues to monitor(step 90) the data subscriber line 14 for an “Exiting Sleep Mode” signalfrom the CPE transceiver (step 96). The CPE transceiver transmits thissignal when its controller receives an “Awaken” indication (step 98)from an external source such as a computer in which it is installed orfrom other sources, or when its controller detects the presence of newdata in the input buffer 18. In response to the “Awaken” signal, the CPEtransceiver retrieves its stored state from the state memory 38;restores full power to its circuitry; and restores the output of the FFT56 to the input of the PLL 62 (step 96). The CO transceiver, ondetecting the “Exit Sleep Mode” notification from the CPE transceiver(step 98), thereupon exits sleep mode by restoring its state andrestoring power. On waking up from sleep mode, the CPE transceiver canbegin transmitting immediately or after only a few frames delay, sinceit need not repeat the initialization that was earlier required toestablish the requisite parameters (e.g., frequency and time-domainequalizer coefficients (FDQ; TDQ, echo-canceller coefficients (ECC),transmitter gains; transmission and reception data rates; transmissionand reception coding parameters; transmission fine gains; and BitAllocation Tables) required for reliable communications. The same istrue for the CO transceiver.

The present invention enables rapid resumption of transmissions, whetherrecovering from a power down or from an enforced idle condition due totemporary unavailability of processor resources in the case of anembedded transceiver, i.e., a transceiver implemented largely softwareand sharing CPU processing power with other applications which may fromtime to time divert CPU resources from the transceiver. In particular,the transceiver of the present invention is capable of recovering fulldata transmission capabilities within a period of a few frames.

On resuming communication, it may be desirable for the CPE transceiverto transmit several frames of test (known) data (step 100) beforeresuming transmission of user data. This enables the system to verifythat system conditions have not changed so significantly as to requirerenewed initialization. If the CO transceiver receives these withouterror, it notifies the CPE transceiver (step 102) and the latter resumesfull user data transmission (step 104 and 106). Otherwise,reinitialization must be performed (steps 104 and 108) before user datatransmission occurs. A similar procedure may be employed for thetransmission by the CO transceiver, and need not be described in furtherdetail.

It will be understood that the order of certain of the steps describedabove may be changed, and that some steps may be omitted or added. Forexample, instead of initiating sleep mode at the CPE transceiver asshown in FIG. 2, the CO transceiver may initiate sleep mode. In such acase, the flow of notifications will be as shown in FIG. 2, but with thepositions of CO and CPE transceivers reversed. Further, in some cases itmay be desirable to omit the preparatory notifications of steps 82 and84 and commence entrance into sleep mode as at steps 86 and 88. Thiswill speed entrance into sleep mode, but is more prone to error.

It should also be understood that it is possible, and in variouscircumstances may be desirable, to operate in a “partial” sleep mode, inwhich only part of each transceiver is powered down. For example, wheredata transfer is one-way (when, for example, receiving video at the CPEtransceiver from the CO transceiver without any upstream data being sentin return to the CO), the CO receiver and the CPE transmitter mayoperate in the sleep mode, while the CO transmitter and the CPE receiverare operating in full power mode.

As noted earlier, it is possible to implement major portions of the CPEand CO transceivers in software. In some applications, a dedicated CPUwill be used for this purpose; in others, the CPU will be shared withother applications. Even when the sleep mode power conservationprocedures described above are not necessary, when the transceivers areimplemented in a shared-CPU environment, it may often be necessary toenter an idle mode that is similar to the sleep mode described above inorder to maintain synchronization between the transceivers whenever theCPU is unable to service the transceivers. This is accomplished inaccordance with the present invention by providing an InterfaceTransmission Unit (ITU) between the CPU and the data subscriber linewhich generates an idle indicator whenever the CPU is unavailable forservicing the transceiver which and transmits it to the remotetransceiver to maintain the latter in synchronism with the former.

In particular, referring now to FIG. 3, an Interface Transmission Unit(ITU) 200 in accordance with the present invention is interposed betweena CPU 202 and the data subscriber line 14. The unit 200 includes a PCIbus interface 204, a data buffer 206, an analog-to-digital anddigital-to-analog converter 208, a controller 210, an idle indicatorgenerator 212, and an analog front end 214.

In normal operation, the CPU passes data and control signals to the ITU200 via the bus interface unit 204. Data from the CPU that is to betransmitted over the line 14 is stored in the buffer 206 and passedthrough the digital-to-analog converter portion of unit 208 forapplication to the line 214 through the analog front end. Conversely,data that is received from the line 14 is passed through thedigital-to-analog converter portion of unit 208 to the buffer 206 andthence to the CPU 202 via the interface 204. The CPU processes the dataflowing in the different directions in the manner illustrated inconnection with FIG. 1, i.e., it performs the requisite IFFT and FFTtransforms on the data as appropriate; The idle indicator generator maybe inactive in this state.

When the CPU must divert its resources elsewhere for servicingapplications having higher priority at the moment, the controller 210activates the idle indicator generator 212 which generates a synchronousidle indicator symbol for transmission to the other transceiver withwhich the present transceiver is communicating. For example, the CPU mayaffirmatively notify the controller 210 that this is about to occur.Alternatively, the controller may monitor the buffer 206 and note itsfilling with data by the CPU has been interrupted for some time period.The idle indicator symbol is preferably of the same duration as a dataframe, so that the remote transceiver can process it in the normalmanner. However, it must be distinct from any valid data frame so thatit can be detected and distinguished without ambiguity. It may comprisea tone, a collection of tones, a collection of tones modulated by bits,with similar of different phases, an inverted sync signal, or even theabsence of signal. The CO transceiver responds to receipt of the idlesymbol by itself transmitting idling data to the network or otherdevices with which it is connected so that erroneous data will not betransmitted. When the CPU returns to servicing its associatedtransceiver, the idle generator suspends operation, preferably oncommand of the controller 210.

Even when the transceiver is not implemented as part of the sharedresources of a CPU, it may nonetheless be beneficial to transmit an idlesignal as described above when the transceiver is in sleep mode. Thisminimizes the likelihood that erroneous data may be emitted, forexample, to a network by a CO transceiver when a CPE transceiver withwhich it is communicating is in sleep mode.

From the foregoing, it will be seen that we have provided a multitonetransceiver system that capable of operating wholly or partially in asleep mode, both for purposes of power conservation as well as toaccommodate itself to integration with, or incorporation into, computersystems having a power conservation mode. The transceiver system also isadapted to implementation in software under circumstances where the CPUon which the software runs is shared with other applications which maysometimes preempt resources needed for the transceiver.

1-17. (canceled)
 18. A multicarrier transceiver having a low power sleepmode capability comprising: means for storing selected state parametersrepresenting characteristics of a communications channel over which thetransceiver is operating; means for reducing power to portions of thetransceiver to enter the low power sleep mode; means for restoring powerto the portions; means for restoring the transceiver from the sleep modebased on the selected state parameters; and means for restoringcommunications without a reinitialization of the transceiver.
 19. Thetransceiver of claim 18, further comprising means for maintaining asynchronized data frame count.
 20. The transceiver of claim 18, furthercomprising a pilot tone that is utilized to maintain synchronizationwith a remote transceiver.
 21. The transceiver of claim 18, furthercomprising means for maintaining synchronization between the transceiverand a remote transceiver.
 22. A low power sleep method for use in amulticarrier communications environment comprising: storing selectedstate parameters representing characteristics of a communicationschannel over which a transceiver is operating; reducing power toportions of the transceiver to enter a low power sleep mode; restoringpower to the portions; restoring the transceiver from the sleep modebased on the selected state parameters; and restoring communicationswithout a reinitialization of the transceiver
 23. The method of claim22, further comprising maintaining a synchronized data frame count. 24.The method of claim 22, further comprising utilizing a pilot tone tomaintain synchronization with a remote transceiver.
 25. The method ofclaim 22, further comprising maintaining synchronization between thetransceiver and a remote transceiver.